Released microstructures are commonly used in a variety of sensors, actuators and other useful devices. Released microstructures are suspended above a substrate (e.g. silicon) to which they are usually attached or anchored. Examples of released microstructures include comb drives, cantilevers, beams, membranes, switches, electrostatic motors and a wide variety of sensors (e.g. pressure sensors, magnetic sensors).
Released microstructures are often made from polysilicon. This is because polysilicon can be conformally deposited on many surfaces and it can be doped to provide conductivity. Also, polysilicon is easily released because there are a number of supporting materials available that can be selectively etched from under or surrounding a polysilicon layer (e.g. phosphosilicate glass, PSG). However, polysilicon has the great disadvantage that deposited polysilicon layers can have relatively high internal stress. Therefore, polysilicon structures tend to distort and bend when released. The tendency of polysilicon to bend after release is undesirable for making precision micromachined structures. Another disadvantage of polysilicon is that it can have a relatively low strength. In addition, polysilicon can have various states of crystallinity—ranging from relatively amorphous to having relatively large crystal grains. The size of these grains and their orientation effects the mechanical properties of the polysilicon layer. Even when polysilicon is deposited with the desired mechanical properties and grain size, the thermal budget in later processing may cause changes in these properties due to grain growth, precipitation at the grain boundaries, and other physical phenomena. For example, doped amorphous polysilicon may become significantly polysilicon during a LPCVD silicon nitride or PSG deposition that occurs after its deposition.
Polysilicon can be annealed to reduce internal stress and reduce bending. However, polysilicon annealing techniques are cumbersome and can interfere with other process steps required in making a useful device. It would be an improvement in the art to obviate polysilicon annealing in making certain released microstructures.
Due to these disadvantages of polysilicon, silicon nitride is sometimes used instead for released microstructures. Low stress silicon nitride films are readily formed without annealing by increasing the silicon content of the film. Also, silicon-rich silicon nitride is rigid, strong, and can be released from a variety of supporting layers. A problem with silicon nitride is that it is an electrical insulator, preventing the ability to charge and discharge the surface as is required in electrostatic actuators. Further, it can not be used alone to conduct a voltage or current as may be desired in electrical switching. For devices that require a thermally conductive released microstructure it may not be desirable or possible to use silicon nitride. Likewise, for devices in which high thermal conductivity is desirable, such as for heat dissipation, it may not be desirable or possible to use silicon nitride.
Therefore, there is a need in the art for an electrically conductive material that can form low internal stress, high strength micro structures. Such a material could be used in a wide variety of released microstructures. In addition, there is a need in certain applications for materials with high thermal conductivity.
U.S. Pat. No. 5,936,159 to Kano et al. discloses a released cantilever having a three-layer structure. The middle layer is a very thin stress relieving layer that tends to equalize stress in the cantilever, thereby reducing bending. In a preferred embodiment, a stress relieving layer tens of angstroms thick is disposed between thicker films of polysilicon.
U.S. Pat. No. 5,475,318 to Marcus et al. discloses a micromachined cantilever probe for contacting integrated circuits. The cantilever has two layers with different coefficients of expansion. When heated, the cantilever bends to provide electrical contact with a nearby electrical pad.
U.S. Pat. No. 5,866,805 to Han et al. discloses a cantilever having a magnetic thin film. The magnetic thin film provides magnetic coupling to a nearby electromagnet. The electromagnet can cause the cantilever to vibrate for use in ‘AC mode’ force microscopy. A second layer is applied to the cantilever to reduce bending of the cantilever.
U.S. Pat. No. 5,796,152 to Carr et al. discloses a cantilever having two separately bendable actuator sections. Each section can be heated separately. In is way, the cantilever can be caused to bend in complex shapes such as S-curves.